When I made the switch from screen several years back, tmux didn't need a patched version to support vertical splits, which was nice. Other things included better support for scripting, nicer configuration, and better mouse support and scrollback behavior by default.

Screen may have caught up a bit since then, but tmux seems to have a more active community and generally faster pace of development.

Fundamentally they do the same thing. TMUX has a nice status bar, it's easier to play around with windows and it's got 256 colours support, as far as I'm aware. But really the difference is in TMUX having a dedicated community while GNU screen is just another GNU utility.

I used to use the Screen as a terminal multiplexer, but for the last 3 years, I have really only used Tmux. It is not better or worse. It's just different :)

I usually choose Tmux over GNU Screen because it seems more intuitive to use IMO, but I still choose Screen for some cases like connecting to serial consoles.

I started using tmux several years ago because it had better script support than screen did, at that time. I also liked the session/pane/window handling better (customization made a lot more sense to me, as a vim user), it supported some plugins that I really liked, and the ability to sync panes was a cool feature that came in handy sometimes (grepping logs for load-balanced applications), but I honestly rarely used.

But I couldn’t tell you now, without some digging, whether screen supports all these features today, or if it’s perhaps even better than tmux.

I'm sure tmux is great. But I've never seen the need to change. I'd happily read an article that shows me the benefits of changing.

Here's my take, most of which is in other comments:

If you like GNU Screen, continue using that, they are very similar tools. I’m a long term user of both, using both for a while, and now only using Tmux exclusively. I need a terminal multiplexer, since I often use my main computer via ssh.

The main advantage of GNU Screen for me was it was slightly more robust, it crashed less for me. While both are pretty stable. I can run both for weeks at a time, and not have any issues. Though, I’ve heard others report the opposite about stability between the two tools.

Tmux has some advantages for me:

   - Tmux is scriptable.  You can run tmux commands to query and set  the state
    of Tmux. It is possible to pipe and process those results. There is some
    thought behind the commands, that make them consistent and orthogonal.
    Screen never really could not do much beyond listing the  available
    sessions.
     - This has made it easy for 3rd party developers to create Tmux plugins, adding some useful features.
  - I think GNU Screen can now do split screens, but I feel it is easier to do
    in Tmux.  Many times I have a monitor with full a window terminal, and Tmux makes
    it easy to split that up into smaller independent terminal panes.

  - GNU Screen development had pretty much stalled for years.  Tmux is being
    actively developed.  Outside developers are welcome to make contributions. 
     - The lead Tmux developer, Nicholas Marriot,  is actively engaged with
       users. He responds to bug reports, helping track down bugs, and
       collaborating with contributors.
     - I believe GNU Screen development has started again, but to me it just
       doesn’t have the same level of engagement from the developers.
     - Tmux has regular releases, compared to Screen
  - Tmux is more current with the latest ANSI/SGR/OSC codes, and has robust
    unicode support.  This hurt me most when I use  a wide terminal (> 256
    columns), and wanted to use the mouse, going past a certain column was
    broken.  There is a new encoding SGR 1006, and it didn’t work with GNU
    Screen, but did with Tmux.   GNU Screen picked this up in 2019, but Tmux
    had it for years before.  There are other codes for 24bit color, italics,
    fancy underlines, etc that GNU Screen doesn’t support, but many modern
    terminal emulators do.  Even ITerm2 has Tmux support builtin. While I
    generally don’t need these but if/when I do, Tmux just works, while GNU
    Screen just ignores these codes.

While both are capable and usable, there are a lot of details that make Tmux better to me.

Screen is fantastic. Yet tmux btought us the future.